A turbine system is typically used to drive a generator, which in turn generate electricity. A turbine typically burns fuel, such as methane, mixed with compressed fluid containing oxidant, e.g., air, resulting in hot combustion gas. The combustion gas impinges on rotor blades and rotates the rotor associated with the blades. The rotary power is transferred to a generator. In such the turbine system, the thermal efficiency can be increased by raising the combustion gas temperature.
Generally, the blades, which can include stator and rotor blades, are made of metal. If metal blades are used, the combustion-gas temperature cannot be too high since high temperatures weaken or melt metal. Thus, the metal blades need to be cooled if the combustion-gas temperature is high, or be exposed to lower (less efficient) combustion-gas temperatures. Some known methods of cooling metal blades include circulating coolant through the blade and blowing coolant from the blade inside to the blade surface. In the latter method, a film of low-temperature coolant is formed on the blade surface to reduce the heat propagating from the combustion gas to the blade.
In spite of these cooling methods, however, there is a limit as to how high the combustion-gas temperature can be. Furthermore, it is difficult to increase efficiency with the above methods, particularly the blowing method of blowing coolant to the blade surface, because the coolant will lower the combustion-gas temperature according to a flow ratio of the coolant.
To enhance efficiency, attempts have been made to recover thermal energy from the hot gas discharged from the turbine system. For example, the hot gas discharged from the turbine is directed to a heat recovery steam boiler, which generates steam under high temperature and high pressure. Steam is supplied to a steam turbine, which in turn generates additional power. Because the heat that would be lost otherwise is recovered, improving the efficiency of the overall system.
In another proposal aimed at improving efficiency, the recovered heat from the exhaust gas heats the fuel supplied to the turbine to improve the chemical energy of the fuel (via reforming process). The major component of natural gas, which is one of the most common fuel, is typically methane. Methane can be reformed chemically by mixing it with steam in a high temperature environment (about 800xc2x0 C.). Methane can be converted into hydrogen and carbon monoxide by maintaining high temperature under an existence of a catalyst, such as nickel.
A turbine system that reforms fuel by using the turbine exhaust gas with methane as a major fuel source is disclosed, for example, in Japanese patent (Koukai) No. 2-286835 and Japanese patent (Koukai) No. 7-269371 (or its U.S. counterpart, U.S. Pat. No. 5,431,007). These Japanese patents disclose about needing to increase the temperature of the turbine exhaust gas, which is typically about 550xc2x0 C., to efficiently reform the fuel. The latter reference discloses heating the exhaust gas temperature to preferably 650xc2x0 C. from its exhaust temperature of about 550xc2x0 C. at the pressure of 7-13.8 ata (0.7-1.38 MPa). The former reference discloses raising the fuel temperature during reformation by burning fuel in an auxiliary combustor and supplying heat to the reformer. The latter reference also discloses an alternative way of raising the fuel temperature during reformation, by circulating steam through the turbine blade. Steam absorbs heat from the turbine blade and cools the same. The steam circulated through the turbine blade, and thus having a higher temperature due to heat absorption from the turbine blade, is then used to heat the fuel in the reformer.
Even with these methods, it is difficult to improve efficiency of the turbine system because the auxiliary combustion consumes fuels in addition to the fuel supplied to the combustor of the turbine. Moreover, the turbine blade is typically heat-resistant to about 800xc2x0 C. Coolant, such as cooling steam running through the turbine blade is below 800xc2x0 C. Therefore, it is difficult to obtain the desired reforming temperature of about 800xc2x0 C. by supplying cooling steam that ran through the turbine blade to the reformer, for example, under pressure of 7-13.8 ata. Thus, it is difficult to improve the fuel reforming ratio.
The present invention relates to a turbine system for producing power and a method thereof. The turbine system includes a reformer, an oxidant compressor, a fuel compressor, a combustor, and a turbine. The reformer reforms fuel containing a mixture of hydrocarbon gas and steam to produce hydrogen and carbon dioxide. The fuel compressor compresses the fuel reformed by the reformer and the oxidant compressor compresses oxidant. The combustor burns the compressed oxidant and the reformed fuel to generate hot gas. The turbine receives the hot gas from the combustor to generate power. The reformer receives the hot gas discharged from the turbine as a heat source to heat the fuel and chemically reform the fuel.
The method of producing power includes reforming a fuel containing a mixture of hydrocarbon gas and steam in a reformer by heating with a heat source to produce hydrogen and carbon dioxide; compressing the reformed fuel and oxidant; generating hot gas by combusting the compressed reformed fuel and oxidant in a combustor; discharging the hot gas to a turbine; and generating power with the turbine. The heat source is the exhaust gas discharged from the turbine.
According to one aspect of the invention, the reformer reforms the fuel under pressure less than or equal to 7 ata.
The turbine system can include a cooler or condenser or both. The cooler cools the reformed fuel with a coolant before the reformed fuel is compressed with the fuel compressor. The condenser extracts water contained in the cooled reformed fuel before compressing the cooled reformed fuel with the fuel compressor.
The coolant can be water or liquid that evaporates. In the case of water, the reformed fuel heats water and generates steam. Heat exchange between water and the reformed fuel cools the reformed fuel. The steam can be introduced directly into the combustor or mixed with the reformed fuel. The reformed fuel mixed with the steam can be compressed with the fuel compressor before it is introduced into the combustor. In the case of evaporative liquid, heat exchange between the reformed fuel and the cooling liquid evaporates the liquid. The evaporated liquid can be introduced directly into the combustor.
According to another aspect of the invention, the amount of power generation can be regulated with a power generation regulator, such as a valve. This can be achieved by regulating the amount of steam to be mixed with the hydrocarbon containing gas.
The turbine system can further include means for raising the reforming temperature. The reforming temperature raising means can comprise an additional combustor that combusts the hot gas discharged from the turbine and additional fuel to raise the temperature of the hot gas introduced to the reformer. The reforming temperature raising means can also comprise oxidant mixed with the pre-reformed fuel. The oxidant causes the pre-reformed fuel to partly combust when the hot gas discharged from the turbine heats the pre-reformed fuel in the reformer, thus raising the reforming temperature. The oxidant can be obtained from the hot gas, such as the hot gas discharged from the turbine, reformer, or evaporator.